Unconscious number discrimination in the human visual system

Humans’ ability to discriminate approximate numerosities is believed to rely on an evolutionarily ancient Approximate Number System (ANS). Measures of the ANS typically rely on explicit judgments of stimulus numerosity. Here we tested for unconscious sensitivity to approximate number by adapting the steady state visual evoked potential (SSVEP) technique. We recorded EEG from human participants (N=21) while they viewed dotclouds flashing at 30 Hz. Each 9.6-second trial consisted of 288 dotclouds. On each trial, half of the dotclouds contained 10 dots (standards), and these standards alternated with other dotclouds whose numbers varied from 10 to 20 dots (oddballs). Within each trial, all oddballs had the same number of dots. Across trials, the standard:oddball ratio ranged from 1.0 (10:10) to 2.0 (10:20), in steps of 0.1. After each trial, subjects judged whether all of the dotclouds had the same number of dots, or if some of them differed. We analyzed EEG spectral power for SSVEPs at 15 Hz – the rate at which numerosity alternated in the stimuli. Results showed that power at 15 Hz was stronger than power at adjacent frequencies, and that this increase in power was positively correlated with the numerical ratio of the stimuli. This effect remained significant when controlling for non-numerical features of the stimuli. Although the neural signal (SSVEPs) showed that participants were discriminating dotcloud ratios implicitly, participants were at chance judging the dotcloud ratios explicitly. Results indicate that people can process visually presented numerosities rapidly and unconsciously.